1. Field of the Invention
The present invention relates to an air-fuel ratio control device of an engine.
2. Description of the Related Art
In the past, to obtain better exhaust emission of internal combustion engines, wide use has been made of the system wherein an air-fuel ratio sensor, for example, an O.sub.2 sensor, is arranged upstream of a three-way catalyst provided in the exhaust system and feedback is given in accordance with the output of the O.sub.2 sensor so that the air-fuel ratio becomes the stoichiometric air-fuel ratio. In such a system, the O.sub.2 sensor is arranged at the convergence portion of the exhaust manifold upstream of the catalyst. Since the O.sub.2 sensor is in the exhaust system near to the combustion chambers, however, (1) the characteristics of the output change along with time due to the heat of the exhaust, the lead in the fuel, etc. and precise detection of the air-fuel ratio of the engine became impossible and (2) since an O.sub.2 sensor is very susceptible to the effects of the air-fuel ratio of any specific cylinder, when the characteristics of the fuel injection valves etc. provided in the cylinders vary or change, it sometimes becomes impossible to suitably detect the air-fuel ratio of the engine as a whole.
Therefore, in the above-mentioned system, the air-fuel ratio sometimes could not be suitably controlled. As a way to solve this, a system has been proposed wherein an O.sub.2 sensor is arranged only downstream of the catalyst or also downstream of the catalyst and the air-fuel ratio is subjected to feedback control using the output of that downstream O.sub.2 sensor as well (see Japanese Unexamined Patent Publication No. 1-203633). The O.sub.2 sensor downstream of the catalyst in this system has a higher precision of detection of the air-fuel ratio of the engine and therefore an extremely good precision of control of the air-fuel ratio of the system due to the facts that (1) there are less changes in characteristics due to heat since the exhaust heat downstream of the catalyst is lower than the exhaust heat just after the combustion chambers as the distance from the combustion chambers to the catalyst is longer, (2) there are less changes in characteristics due to toxication since various toxic substances in the fuel are trapped by the catalyst, and (3) the gas in the cylinders is sufficiently mixed in the catalyst and there is little effect of any specific cylinder.
However, since a three-way catalyst has an O.sub.2 storage function, i.e., absorbs and holds oxygen during lean air-fuel ratio operation when there is a large amount of oxygen in the exhaust gas and discharges the absorbed and held oxygen during rich air-fuel ratio operation, even in this system there are cases where the precision of control of the air-fuel ratio falls under specific operating conditions. That is, in this system, during a fuel cut or a lean air-fuel ratio operation such as when secondary air is introduced into the exhaust system upstream of the catalyst, where the air-fuel ratio upstream of the catalyst continues to be leaner than the stoichiometric air-fuel ratio, feedback control by the output of the O.sub.2 sensor downstream of the catalyst is stopped and at that time a large amount of oxygen is absorbed and held in the catalyst. Next, immediately after the lean air-fuel ratio operation ends, feedback control by the output of the O.sub.2 sensor downstream of the catalyst is started.
The O.sub.2 sensor, however, judges that there is too much oxygen in the exhaust gas and judges that the engine is in a lean operating state when there are oxygen molecules at the sensor detection portion and judges that the engine is in a rich operating state when the oxygen molecules at the sensor detection portion bond with the unburnt component in the exhaust gas and disappear.
If, however, a large amount of oxygen is absorbed and held in the catalyst as mentioned above, even when the air-fuel ratio becomes rich after the lean air-fuel ratio operation has ended, the unburnt components in the exhaust gas will bond with the oxygen absorbed and held in the catalyst and will disappear in the catalyst. As a result, the oxygen molecules adhering to the sensor detection portion of the O.sub.2 sensor during a lean air-fuel ratio operation will continue to adhere to the sensor detection portion as they are without bonding with the unburnt components and therefore even if the air-fuel ratio becomes rich, the O.sub.2 sensor will judge that the engine is in the lean operating state. As a result, the air-fuel ratio will be feedback controlled so as to become further richer and thus the exhaust emission will deteriorate.
In this way, if a large amount of oxygen is absorbed and held in the catalyst, due to the effects of the absorbed and held oxygen, the O.sub.2 sensor will judge that the air-fuel ratio is lean not only when the air-fuel ratio is lean, of course, but also when the air-fuel ratio is rich. In actuality, however, a rich operating state and a lean operating state repeat after a continuous lean operating state ends due to a fuel cut or supply of secondary air, so the oxygen absorbed and held in the catalyst gradually dissipates during this period and once a certain time has elapsed after the end of a continuous lean operating state, the O.sub.2 sensor will detect the accurate air-fuel ratio without being affected by the oxygen absorbed and held at the catalyst. Therefore, to prevent the air-fuel ratio from tremendously deviating from the stoichiometric air-fuel ratio, it is sufficient to restart the feedback control by the O.sub.2 sensor after a certain time after a continuous lean operating state has ended.
The amount of oxygen, however, absorbed and held at a catalyst depends on the extent of deterioration of the catalyst. The more deteriorated the catalyst, the smaller the amount of oxygen absorbed and held in the catalyst. If the amount of oxygen absorbed and held at the catalyst becomes smaller, then time until which the O.sub.2 sensor can detect the correct air-fuel ratio after the continuous lean operating state ends becomes shorter, so the elapsed time depends on the extent of deterioration of the catalyst. Therefore, there is known a system in which the resumption of the feedback control based on the output of the O.sub.2 sensor after the end of a continuous lean operating state is delayed and the delay time is changed in accordance with the extent of deterioration of the three-way catalyst (see Japanese Unexamined Patent Publication (Kokai) No. 1-203633). In this system, while the resumption of the feedback control based on the output of the O.sub.2 sensor is delayed, the amount of correction of the air-fuel ratio is held to the amount of correction of the air-fuel ratio just before the continuous lean operating state.
When holding the amount of correction of the air-fuel ratio to the amount of correction of the air-fuel ratio just before the continuous lean operating state while the resumption of the feedback control by the output of the O.sub.2 sensor is delayed in this way, the delay time of the resumption of the feedback control based on the output of the O.sub.2 sensor becomes longer and therefore the time of deterioration of the exhaust emission becomes longer since time is needed until the resumption of the feedback.